In co-pending U.S. application Ser. No. 563,623 now abandoned assigned to the same assignee as the present invention, there is disclosed an apparatus for stretching a nonwoven web of orientable polymeric material in a station provided with a set of grooved rollers wherein the grooved pattern of the rolls is generally of a sinosoidal wave. In accordance with such co-pending application, the web of synthetic material is stretched in a manner to effect uniform stretching between the bonding points of each individual fiber thereby producing a web of larger dimension in the direction of stretch.
In co-pending application Ser. No. 614,018, now U.S. Pat. No. 4,116,892 granted Sept. 26, 1978 which is a continuation-in-part of the aforementioned application and assigned to the same assignee as the present invention, there is disclosed a process for stretching a substrate of synthetic thermoplastic material selected from the group consisting of a thermoplastic orientable polymer or a blend of a thermoplastic orientable polymer in which there is admixed an incompatible second phase selected from the group consisting of a group of an incompatible polymer or inorganic material. The substrate is stretched in grooved roller pairs by controlling the velocity of introduction of the substrate to maintain the velocity substantially identical to the surface velocity of the roller pair. In accordance with such invention, the blend is stretched to produce an opaque, low density porous sheet.
In both such applications, the grooved roller was manufactured out of a single piece of metal with the grooves being machined into the surface thereof by lathing or embossing processes. While the grooves of each roller initially were perfectly intermeshed when juxtapositioning two of such rollers as a roller assembly in a stretching apparatus, it was noted that distortions resulted after diverse periods of operation. Apparently, certain portions of the grooves became slightly out of register possibly due to irregular stress releases in the metal of the roller sets, or to differences in temperature causing differences in thermal expansion. In the event of very fine stretching grooves (distance between grooves of 0.040 inch or finer), temperature differences of less than 1.degree. C. seem to effect sufficient distortion from one end of a roller to the other, particularly in roller lengths of greater than 40 inches, to cause serious operating problems.
As more clearly hereinafter discussed, while substrates may be uniformly stretched in areas of the roller assembly where intermeshing is perfect, other areas of the surface of the roller assembly may lead to distortion after a period of operation resulting in a cutting action as the roller nip closes. It is believed that a distortion of about 0.001 inch causes sufficient problems of cutting of the substrate being treated to render the resulting product worthless, particularly when using rollers formed with very fine stretching grooves.